Tap changer

ABSTRACT

A tap changer for switching the load current of a transformer from a first winding output to a second winding output includes two pairs of anti-parallel connected semi-conductor rectifiers with a control device for each pair. A switch arrangement is provided for connecting each of the pairs between any of the outputs and a load current connection. The device includes a control arrangement for controlling and performing the switch between the outputs, which includes a device for transmitting the load current from the rectifier pair connected to one winding output to the pair connected to an adjacent winding output. This transfer is carried out partly by natural commutation except when there is a decrease in the turns of the winding and at the same time a resistive load and partly by means of switching when the load current passes through zero upon a decrease in the turns of the winding and at the same time a resistive load.

United States Patent [1 1 [451 Apr. 17, 1973 Elvin [54] TAP CHANGER [75]Inventor: Sten Elvin, Vasteras, Sweden [73] Assignee: Allmanna SvenskaElektriska Aktiebolaget, Vasteras, Sweden [22] Filed: Dec. 23, 1971 [2]]Appl. No.: 211,329

Foreign Application Priority Data Feb. 5, 1971 Sweden ..l4l6/7l 52 us.Cl. ..32a/43.s s [51] Int. Cl .cos: we [58] Field of Search ..323/43.5s. 22 sc,

[56] References Cited UNITED STATES PATENTS 3,619,765 ll/l97l Wood..323/43.5 S 3,437,913 4/1969 Matzl ..323/43.5 S

Primary ExaminerGerald Goldberg Attorney-Jennings Bailey, Jr.

[5 7] ABSTRACT A tap changer for switching the load current of atransformer from a first winding output to a second winding outputincludes two pairs of anti-parallel connected semi-conductor rectifierswith a control device for each pair. A switch arrangement is providedfor connecting each of the pairs between any of the out puts and a loadcurrent connection. The device includes a control arrangement forcontrolling and performing the switch between the outputs, whichincludes a device for transmitting-the load current from the rectifierpair connected to one winding output to the pair connected to anadjacent winding output. This transfer is carried out partly by naturalcommutation except when there is a decrease in the turns of the windingand at the same time a resistive load and partly by means of switchingwhen the load current passes through zero upon a decrease in the turnsof the winding and at the same time a resistive load.

1 Claim, 7 Drawing Figures PATENTED APR 1 71973 SHEET 1 [1F 3 TAPCHANGER BACKGROUND OF THE INVENTION 1. Field of the Invention Thepresent invention relates to a tap changer for switching the loadcurrent of a transformer from a first winding output to a second windingoutput.

2. The Prior Art It is known that closing and breaking processes in tapchangers can be performed by semi-conductor rectifiers, preferablythyristors (IEE conf. I969 publ. 53, part 1, pages 185 192). It is alsoknown that connections between the winding outputs of a transformer canbe carried out without overlap when the load current passes through zero(Siemens Zeitschrift, 39 (1965), Volume 4, page 270) and by means ofnatural commutation (German published specification No. 1,247,479). Inboth cases it is known to use anti-parallel connected thyristors asconnection elements. Two such pairs are necessary per phase. A selectorconnects them to conducting or current winding outputs, respectively,and there are switching contacts to protect the thyristors and avoidconducting losses. The brief conducting interval means that thetransient thermal impedance will be dimensioned for the load current.The voltage between the winding outputs where connection takes place,the step voltage, will be dimensioning. In principle a system entirelywithout movable parts could be used, but this requires one thyristorpair for each winding output.

When the load current is switched at its passage through zero, thethyristor which is conducting at the departing output when the currentpasses through zero is allowed to die. After a short rest the controlcurrent is passed to the appropriate thyristors. During the pause thethyristors are subjected to the entire transformer voltage U, which isassumed to be much greater than the step voltage U,. RC circuits protectagainst over-voltage, that is, take up the temporary load current. Whenthere is a connection towards lower turns of the winding, by which ismeant a decrease in the voltage over the power emitting side of thetransformer in relationto the power absorbing side of the transformer,the departing thyristor receives the returning transformer and stepvoltage in the form of reverse blocking voltage. When there is aconnection towards higher turns of the winding, by which is meant anincrease in the voltage over the power absorbing side of the transformerin relation to the power emitting side of the transformer, thestep-voltage returns in the form of offstate voltage. This means thatthe pause after zero passage is critical with respect to the recoverytime of the thyristor and rapid thyristors are necessary. The method canbe used for both resistive and inductive loads. In the method usingnatural commutation, one thyristor is conducting at the departingoutput. The thyristor having the same conducting direction as that atthe output in question has off-state voltage and is ignited. The currentcommutates over and the departing thyristor receives reverse blockingvoltage. Connection towards higher turns of the winding is done whilecurrent and voltage are in phase and towards lower turns of the windingwhen current and voltage are in counterphase. Thus a connection cannotbe made towards lower turns of the winding by means of naturalcommutation if the load is purely resistive.

SUMMARY OF THE INVENTION The object of the present invention is toeffect a method of connection when switching the load current ofatransformer from a first to a second winding output which willcompletely avoid the above drawbacks.

If the load voltage is deformed when it passes through zero, thedeparting thyristor may be subjected to off-state voltage too early. Anexpansion of the connection process eliminates this risk at zero currentswitch-overs. The changer switches the load current of a transformerfrom a first winding output to a second adjacent winding output andincludes two pairs of antiparallel conducted semi-conductor rectifierswith a control device for each pair. A switch arrangement is providedfor connecting each of the pairs between any of the outputs and a loadcurrent connection. The device includes a control arrangement forcontrolling and performing the switch between the outputs, whichincludes a device for transmitting the load current from the rectifierpair connected to one winding output to the pair connected to anadjacent winding output. This transfer is carried out partly by naturalcommutation except when there is a decrease in the turns of the windingand at the same time a resistive load and partly by means of switchingwhen the load current passes through zero upon a decrease in the turnsof the winding and at the same time a resistive load.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be more fullydescribed with reference to the accompanying drawings in which FIG. 1shows a known embodiment of a tap changer,

FIG. 2 shows a tap changer with an auxiliary circuit,

FIG. 3 shows an example of one embodiment of the tap changer and FIGS.4a to 4d show sensing and controlling means for the device according toFIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following is a descriptionof the connecting process when using natural commutation and connectingtowards higher turns of the winding in connection with FIG. 1.

FIG. 1 shows a tap changer which comprises two antiparallel-connectedthyristor pairs 1 and 2, 3 and 4, respectively, with their connections8, 9 respectively to winding outputs 11 on one winding 10 of atransformer 12. The other connections of the thyristor pair are bothconnected to a load object 7. A control device 6 is arranged to emitcontrol pulses to the thyristors 1, 2, 3 and 4.

Assume that thyristor 3 is conducting and is connected to the departingoutput. Thyristor l, which has the same conducting direction and isconnected to the output in use, has off-stage voltage. It then igniteswhereupon the current commutates over and the departing thyristor 3receives reverse blocking voltage. Connection to higher turns of thewinding is carried out while current and voltage are in phase and tolower turns of the winding when they are in counterphase. If the load isresistive, connection cannot be carried out by means of commutation tolower turns of the winding. In this case the method is used of switchingthe load current at its passage through zero which will now be describedin connection with FIG. 1.

The conducting thyristor l in the first pair of thyristors (thedeparting pair) is allowed to become extinguished when the load current1', passes through zero. After a short interval the control current ispassed to thyristor 4 in the second pair of thyristors (the presentpair). During the interval the entire transformer voltage U, returns,which is much greater than the step voltage U Possibly an RC circuitconnected in parallel with the thyristors may take over the loadcurrent'briefly, to protect against over-voltage. When connectingtowards lower turns of the winding, the returning transformer and stepvoltage is reverse blocking voltage for the departing thyristor 1. Thismeans that only the firing delay determines the length of the interval.When connecting towards higher turns of the winding by means of thismethod, the transformer voltage will return as reverse blocking voltagewhereas the step voltage U, returns as off-state voltage. This meansthat the interval is critical with respect to the recovery time of thethyristors. The method is therefore not suitable for connections towardshigher turns of the winding since it requires extremely high speedthyristors.

If the voltage is deformed at its passage through zero, the departingthyristor can be subjected to off-state voltage too early. This causesno problems when connecting with the method using natural commutation,but otherwise the method illustrated in FIG. 2 can be used in which anauxiliary circuit is used consisting ofa resistor 13 in series with athyristor which is connected in parallel with the present thyristor pairand is used during connection towards lower turns of the winding whenthere is a purely resistive load.

The connection process is now described. Thyristor l is presumed to beconducting. Just before the current passes through zero, the thyristor 5ignites, whereupon a short-circuiting current U,/R flows through theresistor 13, R being the resistance of the resistor. The current throughthe thyristor 1 is then U,/R. Just after the current passes through zerothe thyristor 4 is ignited, whereupon the auxiliary circuit isshort-circuited and the switching is complete. The voltage across theresistor 13 gives a reverse blocking voltage contribution to thethyristor 1 at the same time that the load current i, is taken over bythe thyristor 5 before the thyristor 4 ignites.

FIG. 3 shows a part of one winding of the transformer 12, with itswinding outputs 111 115. The connection contacts 8 and 9 of the tapchanger are connected to the same winding output 113. The tap changercomprises three pairs of antiparallel-connected thyristor pairs 31 and32, 33 and 34, 35 and 36, respectively. The thyristor pair 31 and 32 isconnected between one connecting contact 9 of the load switch and aswitching unit 29 to its contact 43. From the connecting contact 9 thereis another connection 37 to the contact 44 of the switching unit 29.Across the thyristor pair 31 and 32 is a sensing means 50, shown in FIG.4a, connected by way of a winding 21. Between the thyristor pair 31 and32 and the contact 43, the sensing means 50 is connected by way of awinding 25. The thyristors 31 and 32 are controlled by a control device14.

The thyristor pair 33 and 34 is connected between the connecting contact8 and the contact 42 of the switching unit 29. From the connectingcontact 8 there is an additional connection 38 to the contact 41 of theswitching unit 29. Across the thyristor pair 33 and 34 a sensing means60, shown in FIG. 4b, is connected by way ofa winding 23. Between thethyristor pair 33 and 34 and the contact 42 the sensing means 60 isconnected by way ofa winding 27. Across the thyristor pair 33 and 34 isalso connected an auxiliary circuit comprising a resistor 13 in serieswith the antiparalleI-connected thyristor pair 35 and 36. The thyristors33 and 34 are controlled by a control device 15 and the thyristors 35and 36 are controlled by a control device 16. To the control devices14,15 and 16 there are connections from the controlling means 45, 46,47, which are shown in FIGS. 40 and 4d. The connections a f and E-Ttothe controlling means 45, 46 and 47 comprise the outputs of the sensingmeans 50 and 60, respectively, shown in FIGS. 4a and 4b respectively.The sensing means 50 and 60 are connected by their windings 22, 26, 24,28 to the windings 21, 25, 23, and 27, respectively, shown in FIG. 3.The contacts 17, 18, 19, 20 may consist of microswitches operated by thecontact 20. The contact 20 is a boss fixed to the movable contact 30 inthe switching unit 29 which is connected to the load object 7. Theoutputs on the sensing members 50 and 60 give the following information:

a thyristor 31 is in on-state b neither of the thyristors 31 or 32 is inon-state c thyristor 32 is in on-state d thyristor 31 has off-statevoltage and thyristor 32 has reverse blocking voltage e no voltage overthe thyristors 31 and 32 f thyristor 31 has reverse blocking voltage andthyristor 32 has off-state voltage thyristor 33 is in on-state b neitherof the thyristors 33 or 34 is in on-state thyristor 34 is in on-state dthyristor 33 has off-state voltage and thyristor 34 has reverse blockingvoltage no voltage across the thyristors 33 and 34 f thyristor 33 hasreverse blocking voltage and thyristor 34 has off-state voltage.

The following is a detailed description of a tap changer according tothe invention in accordance with FIG. 3 with its sensing and controllingmeans according to FIG. 4, when connected to higher turns of the windingand also to lower turns of the winding with and without an auxiliarycircuit.

Connection to Higher Turns of the Winding The contact 30 is at itsstarting position in contact with the contacts 41 and 44 according toFIG. 3. The connecting contacts 8 and 9 of the tap changer are connectedto the same winding output 113. When connecting to high turns of thewinding, the contact 30 is turned round its axis 39 in the positivedirection (counter-clockwise). When contact is broken between contact 30and contact 44, the connecting contact 9 is no longer alive, since theentire load current i is flowing through the contacts 41 and 30 to theload object 7. The connection contact 9 can then be moved whilecurrentless to the desired winding output, for example 114. Afterturning in the positive direction, the contact 30 will be in contactwith the contacts 41 and 42. The boss 20 fixed to the contact 30 willthus actuate the microswitch 17. Over the microswitch 17, the flipflop(FIG. 4c) and the control device 15, the ignition current is obtained tothe thyristors 33 and 34. When the contact 30 is turned further in thepositive direction, the entire load current will flow through thecontacts 42 and 30. When the contact 30 is turned 180 it is in contactwith the contacts 42 and 43 and the boss will actuate the microswitch18. By means of the flip-flop 100, the microswitch 18 blocks the controlcurrent to the thyristors 33 and 34. Gate 101 (or 102) will change overwhen the conditions for this are fulfilled, that is, the control currentto the thyristors 33 and 34 is blocked, thyristor 33 (or 34) is inon-state, thyristor 31 (or 32) has off-state voltage and thyristor 32(or 31) has reverse blocking voltage. Natural commutation can then beperformed. The flip-flop 103 (or 104) ignites thyristor 31 (or 32) uponwhich the commutation is carried out. Switching is then completed bygate 105 (or 106). The contact 30 is then turned back to the startingposition in the positive direction, during which time the connectingcontact 8 may also be connected in currentless state to the windingoutput 114.

Connection to Lower Turns of the Winding The contact 30 is in itsstarting position according to FIG. 3 and the connecting contacts 8 and9 are at the same winding output 113. The contact 30 is turned innegative position (clockwise). When the contact between contacts 30 and41 has been broken, the connecting contact 8 can be moved in currentlessstate to the desired winding output, for example 112. The load current ithen flows through the contacts 44 and 30. The contact 30, after beingturned 90, is then in contact with the contacts 44 and 43. The boss 20activates the microswitch 19. Over the microswitch 19, flip-flop 128 andcontrol device 14, ignition current is obtained to the thyristors 31 and32. If the load object 7 is not purely resistive, that is if theconditions for change-over of the gate 120 are not completely fulfilled,the switching is performed by natural commutation as is desired inconnection with connection towards high turns of the winding by means ofgates 130, 131, 134 and 135 and flip-flops 132 and 133. In order for theflip-flop 120 to close, the control pulses to the thyristors 31 and 32must be blocked, no voltage must prevail over the thyristors 31 and 32and neither of the thyristors 32 and 34 may be in on-state. If theseconditions are fulfilled, there will be common zero passage for the loadcurrent i and the load voltage U,. The voltage sensed by the sensingmember 60 is equal to the step voltage U,. The step voltage U, is inphase with the transformer voltage U,. This means that we have resistiveload since the load current and the transformer voltage, that is, theload voltage, are in phase with each other. When the gate 120 switches,there is an input signal to the controlling means 47 through theconnection 129. The thyristor pair 31 and 32 are reignited over theflip-flop 122 and the control device 14. Just before the next zeropassage, the thyristors 3S and 36 are given an ignition pulse throughthe time delay circuit 125, the flip-flop 124 and the control device 16.The time delay 1, is slightly less than a half period. At the same timethe control current to the thyristors 31 and 32 is blocked by theflip-flop 122. Just after the zero passage, the thyristors 33 and 34 aresupplied with ignition current with the help of the time delay circuit123, the flip-flop 126 and the control device 15. At the same time hecontrol current to the thyristors 35 and 36 is blocked by the flip-flop124. The time delay 1', is longer than a half period. The switchover towinding output 1 12 is thus complete. If there is not an auxiliarycircuit, the changeover at zero passage is carried out by shutting thegate 120, by way of the connection 127 to the flip-flop 121 connected tothe control device 15.

The connecting contacts 8 and 9 are movable manually or preferably withmeans controlled by the position of the contact 30.

By means of the invention a tap changer is obtained which in allsituations, regardless of connection towards higher or lower turns ofwinding and irrespective of the nature of the load, can safely andreliably perform switchings.

I claim:

1. Tap changer for switching the load current (i of a transformer (12)from a first winding output (113) to a second winding output (112, 114),comprising a first and second pair of anti-parallel-connectedsemiconductor rectifiers (31,32 and 33,34), a control device (14,15) foreach pair (31,32 and 33,34), means for connecting each of said pairs(31,32 and 33,34) between any of said winding outputs (112,113,114) anda connection for the load current (i,), said tap changer comprisingcontrolling means (45,46) to control and perform the switching from saidfirst winding output (113) to said second winding output (112,114)including means to transfer the load current (i,) from the rectifierpair (31,32 (33,34) connected to the first winding output (113) torectifier pair (33,34 (31,32) connected to the second winding output(112, 114) partly by means of natural commutation except when there is adecrease in the turns of the winding and at the same time a resistiveload (7) and partly by means of switching when the load current (i,)passes through zero upon a decrease in the turns of the winding and atthe same time a resistive load (7), said tap changer including sensingmeans (50,60) to sense current and voltage in each pair ofanti-parallel-connected semiconductor rectifiers (31, 32 and 33,34), anauxiliary circuit comprising aresistor (13) and an additional pair ofanti-parallel-connected semiconductor rectifiers (35,36) in series withsaid resistor (13) parallel-connected with one of said pairs ofsemiconductor rectifiers (31,32 or 33,34), a control device (16)connected to the additional pair of rectifiers (35,36) forming part ofthe auxiliary circuit, a control means (47) including means operableupon switching from the rectifier pair (31,32 (33,34) not connected tothe load current (i,) to the other rectifier pair (33,34 (31,32) toignite the rectifier (35 or 36) in the additional rectifier pair (35,36)which has the same conducting direction as the rectifier (31 or 33) (32or 34) in the rectifier pair (31, 32 (33,34) to which the auxiliarycircuit is parallel-connected, which is arranged to take over the loadcurrent (i

1. Tap changer for switching the load current (i1) of a transformer (12)from a first winding output (113) to a second winding output (112, 114),comprising a first and second pair of anti-parallel-connectedsemiconductor rectifiers (31,32 and 33,34), a control device (14,15) foreach pair (31,32 and 33,34), means for connecting each of said pairs(31,32 and 33,34) between any of said winding outputs (112,113,114) anda connection for the load current (i1), said tap changer comprisingcontrolling means (45,46) to control and perform the switching from saidfirst winding output (113) to said second winding output (112,114)including means to transfer the load current (i1) from the rectifierpair (31,32 (33,34) ) connected to the first winding output (113) torectifier pair (33,34 (31,32) ) connected to the second winding output(112, 114) partly by means of natural commutation except when there is adecrease in the turns of the winding and at the same time a resistiveload (7) and partly by means of switching when the load current (i1)passes through zero upon a decrease in the turns of the winding and atthe same time a resistive load (7), said tap changer including sensingmeans (50,60) to sense current and voltage in each pair ofanti-parallel-connected semiconductor rectifiers (31, 32 and 33,34), anauxiliary circuit comprising a resistor (13) and an additional pair ofanti-parallel-connected semiconductor rectifiers (35,36) in series withsaid resistor (13) parallelconnected with one of said pairs ofsemiconductor rectifiers (31,32 or 33,34), a control device (16)connected to the additional pair of rectifiers (35,36) forming part ofthe auxiliary circuit, a control means (47) including means operableupon switching from the rectifier pair (31,32 (33,34) ) not connected tothe load current (i1) to the other rectifier pair (33,34 (31,32) ) toignite the rectifier (35 or 36) in the additional rectifier pair (35,36)which has the same conducting direction as the rectifier (31 or 33) (32or 34) in the rectifier pair (31, 32 (33,34) ) to which the auxiliarycircuit is parallel-connected, which is arranged to take over the loadcurrent (i1).